[R] Time zones in POSIClt objects

[Jeff Newmiller]

POSIXt vectors do not support different time zones element-to-element.

If you want to keep track of timezones per element, you have to create a vector of timestamps (I would recommend POSIXct using UTC) and a parallel vector of timezone strings. How you manipulate these depends on your use cases, but from R's perspective you will have to manipulate them element-by-element.

I complained about this on this list a couple of decades ago, and was chastised for it. Evidently handling timezones per element was considered to be too impractically slow to be a standard feature.

On October 10, 2024 6:46:19 AM PDT, Jan van der Laan <rhelp using eoos.dds.nl> wrote:
>
>It is not completely clear to me how time zones work with POSIXlt objects. For POSIXct, I can understand what happens: time is always stored in GMT, the `tzone` attribute only affects how the times are displayed. All computations etc. are done in GMT.
>
>POSIXlt objects have both a `tzone` attribute and a `zone` field. It seems that the `zone` field is largely ignored. It only seems to be used for displaying the times, but does not seem to play a role when doing arithmetic and conversions of the times.
>
>For example below, we have the same times in two different time zones. The following seems to do what I expect: when we subtract the two times we get the difference in time between the two time zones:
>
>t1 <- as.POSIXlt(c("2024-01-01 12:30", "2024-01-01 12:30"), tz = "GMT")
>t1$zone
># [1] "GMT" "GMT"
>
>t2 <- as.POSIXlt(c("2024-01-01 12:30", "2024-01-01 12:30"))
>t2$zone
># [1] "CET" "CET"
>
>t1 - t2
># Time differences in hours
># [1] 1 1
>
>
>When I change the `tzone` attribute of t1 to that of t2:
>
>attr(t1, "tzone") <- attr(t2, "tzone")
>t1
>#[1] "2024-01-01 12:30:00 GMT" "2024-01-01 12:30:00 GMT"
>
>The times are still displayed as being in GMT, however when I take the difference:
>
>t1 - t2
>#Time differences in secs
>#[1] 0 0
>
>We get a difference of 0. So it seems that the difference is only based on the `tzone` attribute. The value of `zone` is completely ignored.
>
>I am aware of the following remark in ?POSIXlt on arithmetic operations
>| Be aware that ‘"POSIXlt"’ objects will be interpreted as being in
>| the current time zone for these operations unless a time zone has
>| been specified.
>
>but this does not explain this, I think.
>
>One of the reasons, I ask, is that I have (potentially) times in different time zones. Using POXIXlt objects seems like they could store/support this. But working with this seems unpractical as the `zone` field does not seem to do anything:
>
>t1$zone <- c("CET", "GMT")
>t1 - t2
>#Time differences in secs
>#[1] 0 0
>
>Also the `gmtoff` field does not seem to do anything. For what/where is this field used?
>
>t1$gmtoff <- c(3600, 0)
>t1
>#[1] "2024-01-01 12:30:00 CET" "2024-01-01 12:30:00 GMT"
>
>t1 - t2
>#Time differences in secs
>#[1] 0 0
>
>as.POSIXct(t1)
>#[1] "2024-01-01 12:30:00 CET" "2024-01-01 12:30:00 CET"
>
>So, I am not sure what purpose the zone and gmtoff fields have. Do they have a purpose? Am I using them wrong? The reason I am asking, is that I have some times in potentially different time zones. The data I get is something like:
>
>times <- list(
>  year = c(2024L, 2024L),
>  month = c(1L, 1L),
>  day = c(1L, 1L),
>  hour = c(12L, 12L),
>  minutes = c(30L, 30L),
>  seconds = c(0, 0),
>  timezone = c("", "GMT")
>)
>
>I am looking for ways to convert this into a practical date format for working with in R. Possible time zones are only local time or UTC/GMT. I would be fine with either converting local time to GMT. What would be a good way to convert these to a format R can work with?
>
>Thanks for the help.
>
>Jan
>
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-- 
Sent from my phone. Please excuse my brevity.